Mechanisms governing initiation of first ovulation are being investigated using the sheep as the experimental model. Because in the sheep changes in negative feedback inhibition of gonadotropin secretion are not temporally related to onset of ovarian cyclicity, and because preovulatory surges of gonadotropins occur immediately prior to the first ovulation, studies are being conducted to examine the development of the surge mode of gonadotropin secretion. The following have been proposed as mechanisms whereby the first preovulatory surge of gonadotropins is initiated: (a) the hypothalamo-hypophyseal system becomes responsive to the positive feedback action of 17 beta-estradiol; (b) the ovary becomes capable of secreting 17 beta-estradiol in quantities sufficient to trigger the preovulatory discharge of gonadotropins. The current studies are directed toward determining which of the two mechanisms, neural or ovarian, is the final system to develop, and thus, which is the limiting system in the onset of ovarian cyclicity. BIBLIOGRAPHIC REFERENCES: Foster, D.L., Jaffe, R.B., and Niswender, G.D: Sequential patterns of circulating LH and FSH in female sheep during the early postnatal period: Effect of gonadectomy. Endocrinology 96:15, 1975. Foster, D.L., Lemons, J.A., Jaffe, R.B., and Niswender, G.D.: Sequential patterns of circulating LH and FSH in female sheep from early postnatal life through the first estrous cycles. Endocrinology 97:957, 1975.